1. Field of the Invention
The present invention relates to a novel alkylidenenorbornene polymer excellent in heat resistance, light resistance, transparency and moisture resistance and useful as an optical material and to a process for producing the same.
2. Related Art
Optical polymeric materials mainly used up to now are poly(methyl methacrylate) and polycarbonate. These polymers, however, have drawbacks in that the former shows high water absorption and insufficient heat resistance, while the latter gives injection moldings with high birefringence. Accordingly, it is increasing difficult for these polymers to meet the requirements for optical polymeric materials which are becoming increasingly severe.
As polymeric materials improved in such drawbacks, there have been developed polymers which use polycyclic norbornene-group monomers. For example, it has been disclosed in Japanese Patent Application Kokai (Laid-open) Nos. 60-26,024, 64-24,826, 60-168,708, 61-115,912, 61-120,816 and others that thermoplastic saturated norbornene polymers as hydrogenation products of ring-opening polymers of norbornene monomers and addition-type copolymers of norbornene monomers with ethylene have excellent characteristic properties as optical disk substrate materials.
In the prior arts mentioned above, polycyclic monomers such as tetracyclododecenes must be used as the norbornene monomer to secure high heat resistance required for optical materials. However, the preparation of polycyclic monomers is not always easy. The polycyclic monomers are usually synthesized by addition reaction of norbornenes with cyclopentadiene under heating. While they must be separated and purified by means of distillation since the reaction product contains a substantial amount of byproducts such as oligomers of cyclopentadiene, the separation and purification can be performed only with difficulty. Consequently, polycyclic monomers, particularly tetracyclododecene derivatives, are very expensive and not advantageous for industrial use. Also, they are restricted as to the kinds of available monomers.
On the other hand, it is described in La Chimica et l'Industria, 45, 1,529 (1963) and Macromolecular Chemistry, Rapid Communication, 1, 467 (1980) that, among bicyclic norbornenes, 5-methylidenenorbornene, which has a methylidene group, undergoes transannular polymerization to give a saturated linear polymer. However, 5-methylidenenorbornene used as the monomer is still more difficulty available than said tetracyclododecenes and further has the drawback of being apt to form gels in polymerization. Moreover, the polymer described in La Chimica et l'Industria, 45, 1529 (1963) is crystalline and lacking in optical isotropy, so that it cannot be used as an optical material.